A linear amplifier produces an output signal which is proportional to the input signal in amplitude. If an amplifier provides inadequate linearity, the performance of the amplifier may be degraded by distortion components in the output signal. For example, distortion components may represent intermodulation products formed during the amplification process as different frequency components of the input signal interact with each other. Non-linear performance of an amplifier becomes a particularly acute problem where the amplifier is driven near saturation. If a power amplifier of a wireless communications system is driven near saturation, the bandwidth of the amplified signal may expand, causing adjacent channel interference. Thus, power amplifiers for wireless communications systems, such as personal communication services (PCS) systems, characteristically require some form of linearization to enhance the linear performance of the power amplifier.
The linearity of power amplifiers in a wireless system have been enhanced by feed forward, data pre-distortion, and signal pre-distortion techniques. The feed forward technique of linearization extracts distortion products from an output signal of a main amplifier. A corrective amplifier amplifies a phase-shifted version of the distortion products. The amplified, phase-shifted version is combined with the output signal to cancel out the distortion products from the output signals. However, the feed forward technique can require complex circuitry with extra components and critical adjustments that may tend to reduce actual circuit performance from theoretical performance.
Data pre-distortion considers a data constellation of input data modulating the input signal fed into a power amplifier. The data constellation is pre-distorted in order to compensate for any distortion subsequently introduced by the power amplifier. However, data pre-distortion techniques depend on the modulation scheme so that, for each different modulation scheme the algorithm must be adjusted or re-written to deliver appropriate improvements in the linearization.
The signal pre-distortion method generates a pre-distorted signal for introduction to the input of a primary amplifier. The primary amplifier produces internal nonlinear distortions, which are canceled by the pre-distorter to yield a relatively undistorted signal at the output of the primary amplifier. the signal pre-distortion technique relies on the assumption that the primary amplifier and a secondary amplifier operate in a sufficiently equivalent manner to provide adequate cancellation of the nonlinear distortion. The ideal pre-distortion signal would have a distortion signal component that is inverted in phase, but otherwise perfectly identical to the distortion component introduced by the other amplifier. However, in practice the primary and secondary amplifiers tend to have slightly different characteristics that result in incomplete cancellation of nonlinear products of the primary amplifier. Examples of different characteristics that may detract from linearity-enhancing techniques include gain versus frequency, phase linearity, temperature variation, amplitude modulation/phase modulation conversion or other operational disparities between the amplifiers.
The differences between the amplifiers in the signal pre-distortion technique and feed-forward technique may become apparent when a power amplifier is fluctuated over a temperature range or cycled over a range of different input magnitudes. When co-dependent amplifiers for linearity enhancement operate under extreme environmental conditions or at the fringes of electrical specifications, the linearity of a power amplifier assembly may be degraded. Thus, a need exists for an amplification system that provides linear performance characteristics over various extended temperature and dynamic input ranges of input signals.